Aluminum alloys have been widely used in structural, architectural, and aesthetic applications varying from sophisticated aerospace structures to architectural and furniture applications and even sports applications. Extrusions are a common form of aluminum alloy product as are rod products produced by extrusion with or without subsequent hot rolling and then some cold finishing such as cold drawing. Such rod shapes are generally round but other shapes can be used and find wide application in a number of industries.
A popular family of aluminum alloys is designated as 6XXX alloys in accordance with the Aluminum Association designation system wherein the 6XXX alloys refer to aluminum alloys containing magnesium and silicon as their major alloying additions. It is common in these alloys to include other elements such as chromium or manganese or copper or combinations of these and other elements. One very good example of 6XXX alloys is alloy 6013 which is described in U.S. Pat. No. 4,589,932, the entire contents of which are fully incorporated herein by reference. The Aluminum Association limits for alloy 6013 are 0.6 to 1% silicon, 0.8 to 1.2% magnesium, 0.6 to 1.1% copper, 0.2 to 0.8% manganese, 0.5% maximum iron, 0.1% maximum chromium, 0.25% maximum zinc, 0.1% maximum titanium, other elements 0.05 each, 0.15% total, the balance substantially aluminum. This alloy has performed very well in a number of applications, but it has been found that in certain production processes such as extruded and cold finished rod, large recrystallized grain sizes can occur in the final product which detract from strength properties by lowering them and making them inconsistent from extrusion to extrusion and even along the length of a given extrusion whereby different strength properties may prevail between the front, middle, and back sections of a product. Also, after anodizing or even etching such a product, the large grain pattern results in an undesirable appearance which detracts from product acceptance. If the extrusion stock (billets or castings used for extruding into extrusions) is relatively small, such as 6 to 8 or 9 inches diameter, or the resulting extrusion is quite thin, such as 1/8 or 1/4 inch (or both), such can favor achieving finer grains than using larger commercial scale practices such as ingot or extrusion stock over 11 or 12 inches, such as 13-inch or 14-inch or 15-inch to 18 or 20 or 22-inch or larger diameter stock, and making extruded sections thicker than 1/4 or 1/2 inch (for instance, above 1 or 11/4 inches thick) where, it has been found, the extrusion operation can lead to excessively large recrystallized grains later in the overall manufacturing process. In referring herein to thickness, such as thickness of an extrusion or extruded and drawn product, such refers to the smallest cross section dimension. For a round cross section, it is the diameter; for a rectangle, it is the smaller side.
In making cold finished wire, rod, and bar, a manufacturing method includes preheating or homogenizing an ingot or billet of the alloy which is followed by extrusion at elevated temperature. Extrusion can be followed by hot rolling to further reduce the size of the extruded section. For instance, a 6x6 extrusion can be hot rolled into a 3/4inch round section. After this, the greatly elongated product is solution heat treated and quenched. A drawing or other cold working operation may precede the solution heat treatment, if desired, for instance as part of an overall size reduction schedule. Solution heat treatment for this alloy typically involves very high temperatures, such as temperatures above 1000.degree. F. and typically results in a recrystallized grain condition which is undesirable if the grains are excessive in size which unfortunately is often the case. Following quenching, a cold finishing operation is employed such as by cold drawing to a reduction in size of typically about 10 or 20% after which the product is artificially aged to strengthen it.
In accordance with the invention, it has been found that controlling the extrusion temperature can actually overcome the problem of excessive grain sizes after solution heat treating. That is, extrusion temperatures of 850.degree. or 1000.degree. F. (as referred to in U.S. Pat. No. 4,589,932) can be detrimental in making certain cold finished rod, wire and bar, whereas extruding at temperatures of 400.degree. to 600.degree. F. can be quite beneficial in producing certain products of fine recrystallized grain size.